Quantum effects in cold and controlled molecular dynamics

The wave nature of matter emerges at very low energy or very short timescales. In my talk I will present two such examples from molecular physics – quantum scattering resonances in low-energy collisions and quantum control of photoelectron circular dichroism (PECD) in the ultrafast ionization of chiral molecules. Both examples testify to the importance of theory and experiment evolving together.

In low-energy collisions, quantum scattering resonances are a sensitive probe of the intermolecular forces and may dominate the final distribution of quantum states even for strong and highly anisotropic interactions. This has recently been observed for Feshbach resonances populated in the Penning ionization of dihydrogen by a metastable rare gas atom [1]. For such a small collision complex, full quantum scattering calculations can be carried out [1]. The theoretical predictions for the cross section involve then only the approximations made when constructing the potential. Changes in the shape of the potential thus translate directly into modifications of the cross sections. This can be used to to improve calculated potentials, starting from the experimental data [2]. Conversely, one can also ask by how much the experimental resolution of measured cross sections must improve in order to unambiguously discriminate predictions derived from different levels of advanced ab initio electronic structure theory. 

Moving from low energy to short time scales, I will discuss the quantum control of photoelectron circular dichroism (PECD) of chiral molecules. It refers to an asymmetry that arises between left-handed and right-handed molecules when they are photoionized. Quantum control, i.e., constructive and destructive interference between different excitation pathways, allows for enhancing the chiral observable [3]. The interference can be induced by suitably shaped ionization pulses. PECD, remarkably, requires light-matter interaction only in the electric dipole approximation even for randomly oriented molecules. This results not only in a very large dichroic effect, but provides also a recipe for how to induce and subsequently probe chiral dynamics in initially achiral molecules [4]. The preparation of chiral superposition states with a preferred handedness may be useful in future experiments, e.g. on measuring parity violation with chiral molecules. 

References 

1. Margulis et al., p. 77, vol. 380, Science (2023).
2. Horn et al., eadi6462, vol. 10, Science Advances (2024).
3. Goetz et al., p. 013204, vol. 122, Physical Review Letters (2019).
4. Tikhonov et al., eade0311, vol. 8, Science Advances (2022).